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Fig. 2. A century of changes in organic C concentrations for the plow layer in Morrow Plots cropped to continuous corn [C-C], a corn–oats (1876–1966) or corn–soybean (since 1967) rotation [C-O(S)], or a corn–oats–alfalfa hay rotation [C-O-H]. Open circles represent NB subplots with fertilizer and aboveground residue inputs beginning in 1955. Solid circles represent SA subplots with manure, limestone, and rock phosphate inputs before 1967, subsequently replaced by high NPK fertilization and aboveground residue return. Shaded boxes indicate the introduction of NPK fertilization. Triplicate analyses for organic C were performed by the method of Mebius (1960). Using the Tukey-Kramer statistic at 
= 0.05, significant differences must exceed 2.5 g kg–1 within a time series and 2.9 g kg–1 for comparison of individual values within a chart.
